Chinese authorities have issued a 20-day deadline for Microsoft to explain “compatibility problems” with its Windows and Office software, after previously warning the company not to obstruct an anti-monopoly investigation.

China’s State Administration for Industry and Commerce (SAIC) posted an online notice Monday demanding that Microsoft offer a written explanation within the allotted time. The antitrust regulator has been investigating the company on industry complaints that Microsoft software has “not been fully open,” resulting in compatibility and bundling issues with the products.

The notice was issued a week after SAIC claimed that Microsoft had not been fully transparent in its investigation. The regulator has been questioning Microsoft officials and in July conducted several raids of company offices in China to obtain information on contracts, financial statements, and internal emails.

The Chinese regulator has yet to fully elaborate on the investigation’s specifics. Windows is the country’s dominant PC operating system, but for years Microsoft has been fighting piracy in the China, including by filing lawsuits against alleged offenders.

A Chinese procurement center banned certain purchases of Windows 8 devices in May, with state-controlled media claiming the action was related to security concerns with the software.

China is also investigating other foreign companies on anti-monopoly charges. Among them is Qualcomm, which has allegedly been overcharging clients to use its patents.

Microsoft did not immediately respond to a request for comment. But last week, the company said it was serious about complying with Chinese laws and committed to addressing SAIC’s questions.